The invention relates to a fluid level sensor positioned in a container that dispenses fluid used in forming electronic devices.
In the exemplary manufacturing process of electronic devices, different types of materials are used. For example, adhesives are commonly used to attach one component of a device to another component. To make an integrated circuit device, a die is mounted to a support structure by means of adhesives, which can be one of several types, including metal alloys (e.g., solder) and organic or inorganic adhesives (e.g., epoxies and polyimides, which can be filled with metal).
In the integrated circuit device 10 shown in FIG. 1, a die 14 can be mounted to a leadframe 12 by using an adhesive layer 16 to attach the underside 34 of the die 14 to the top surface 30 of leadframe 12. If an epoxy or polyimide type adhesive is used, the adhesive is typically dispensed in fluid form onto portions of the top surface 30 of the leadframe 12. After formation of the adhesive layer 16, the die 14 is contacted to the adhesive layer 16 to bond the die 14 to the leadframe 12. Next, bond pads 24 on the die 14 are wirebonded (using wires 26) to corresponding bond pads 22 on leadfingers 20 of the leadframe 12. The leadframe/die assembly is then encapsulated using an encapsulant 18 (e.g., plastic).
An adhesive originally in fluid form is dispensed onto a target support structure, such as the leadframe 12 in FIG. 1, using a dispensing container in a die attach machine. Various automated methods have been used to detect the level of the fluid adhesive inside the dispensing container. One method that has been used is magnetic sensing. The dispensing container includes a ferromagnetic coated piston that sits on top of the fluid adhesive. As the adhesive is being dispensed, the level of the fluid decreases, and the ferromagnetic piston falls inside the cylindrical container with the fluid level. When the ferromagnetic coated piston reaches a predetermined position in the container, it activates a magnetic sensor to stop the die attach machine. However, certain types of adhesives contain ferromagnetic fillers that may interfere with the magnetic sensing mechanism.
Various fluid level sensors and detectors have been proposed to detect different types of fluid (e.g., gasoline, water, oil) stored in different types of containers. Such sensors include optical sensors and electrical sensors.
Generally, the invention is directed to a sensor used in a fluid dispensing container, the sensor including a pair of electrical conductors positioned in the dispensing container to which a voltage to detect the amount of fluid in the container.
The invention has one or more of the following advantages. Accurate sensing of the fluid level in a fluid dispensing container can be performed with a large variety of adhesives, including adhesives filled with a ferromagnetic filler. The fluid level sensing can be accomplished without the use of moving parts inside the container, which improves reliability and facilitates handling.
In general, in one aspect, the invention features a system for dispensing an adhesive onto an electronic device. The system includes a container storing the adhesive and a port through which the adhesive can flow. Electronic conductors are positioned in the container. A voltage is applied across the conductors so that a current flow is induced through the adhesive in the container. The current is measured to determine a level of the adhesive in the container.
In general, in another aspect, the invention features a method of applying an adhesive onto an electronic device. The adhesive is stored in a container having electrical conductors positioned in the container. A voltage is applied across the conductors. The current flow through the adhesive is measured to determine the adhesive level in the container. The adhesive is supplied to the electronic device until the adhesive level drops below a predetermined amount.
Other features and advantages will become apparent from the following description and from the claims.